Fix buffer underrun (invalid read) triggered during diagnostic rendering.  The test would overflow when computing '0 - 1'.

I don't have a good testcase for this that does not depend on system headers.
It did not trigger with preprocessed output, and I had trouble reducing the example.

Fixes <rdar://problem/13324594>.

Thanks to Michael Greiner for reporting this issue.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177201 91177308-0d34-0410-b5e6-96231b3b80d8